1. Field of the Invention
The present invention relates to a selection module for an optical signal switch.
2. Description of the Prior Art
Telecommunications is expanding greatly. More and more users (private persons and companies) are transmitting over telecommunication networks more and more messages containing more and more information, for example when sending pictures. To satisfy this growing information bit rate demand, telecommunication network operators are adopting optical signal transmission involving modulation of optical signals, generally produced by lasers, in accordance with the information to be transmitted, followed by propagation of the modulated signals over a network of optical waveguides or optical fibers.
Optical signal transmission has a number of advantages. In particular, optical signals are attenuated less during transmission than electrical signals and optical fibers are mechanically stronger and lighter than their electrical counterparts. However, the main advantages are the high bandwidth of optical fibers and the ability to transmit a plurality of carriers with different wavelengths simultaneously in the same fiber. This latter technique, known as wavelength division multiplexing, provides information bit rates of the order of 1 gigabit/s and even 1 terabit/s.
In parallel with wavelength division multiplexing, time division multiplexing enables the simultaneous transmission of a plurality of calls on the same carrier because each carrier transmits packets relating to different messages whose information content has been divided between the packets, which are sent over the network with a header indicating their destination. When the packet passes through a switch, the latter locks its physical resources for the time needed to route the packet to the requested output. Those resources are then freed to switch another packet. As the packets have a limited time duration, of the order of 1 microsecond, many calls can be transmitted in a short time period. This routing policy is currently used by the largest Internet Protocol networks.
The multitude of signals transmitted by the networks necessitates the introduction of switches for directing the signals received at a plurality of inputs, corresponding to a first set of optical fibers, to a plurality of outputs, corresponding to a second set of optical fibers. By controlling them appropriately, some switches, known as broadcast and select switches, broadcast a signal coming from an input and select the output of the switch to which the signal is sent.
FIG. 1 shows a broadcast and select switch 10.
The switch 10 comprises:                Pe input ports each connected to an input fiber Ij (j varying from 1 to Pe),        Pe demultiplexers DI1 to DIPe,        L×Ce optical regenerators RIj (i varying from 1 to L×Ce),        L multiplexers MI1 to MiL,        L amplifiers A1 to AL,        L couplers D1 to DL,        Cs×Ps selection modules S1 to SCs×Ps,        Cs×Ps optical regenerators ROk (k varying from 1 to Cs×Ps),        Ps multiplexers MO1 to MOPs, and        Ps output ports each connected to an output fiber Om (m varying from 1 to Ps).        
Each input port receives on one of its input fibers Ij a wavelength division multiplex (WDM) signal.
The WDM signals are demultiplexed by the Pe demultiplexers DI1 to DIPe.
The demultiplexed signals are then regenerated by the optical regenerators RIj, which amplify and reshape the optical pulses of the signals. The optical regenerators RIj, can also modify the wavelength associated with each signal.
The regenerated signals are then multiplexed by the L multiplexers MI1 to MIL, each multiplexer having Ce inputs and supplying at its output a signal Ft (t varying from 1 to L).                a plurality of optical switches such as semiconductor optical amplifiers each having an input connected to an output of the demultiplexer and an output connected to an input of the multiplexer.        
Referring to FIG. 1, each of the Cs×Ps selection modules S1 to SCs×Ps selects a particular signal Ft and extracts therefrom a particular channel that is sent to one of the Cs×Ps optical regenerators ROk.
The channels intended for an output fiber Om (m varying from 1 to Ps) are first multiplexed by one of the Ps multiplexers MO1 to MOPs each having Cs inputs and an output connected to one of the Ps output ports.
Certain problems are encountered with a switch of the above kind, however.
The switch implies a large number of optical gates for implementing the selection modules. Each of the Cs×Ps output gates ahead of multiplexing toward the output fibers implies a selection module, and thus a spatial selector and a spectral selector for selecting a channel, both these selectors comprising a large number of optical switches. This large number of optical switches implies not only high cost but also high power consumption and a large overall size of the switch.
The present invention aims to provide a selection module for switching optical signals whereby the number of optical gates used in said switch can be reduced and the overall power consumption and the overall size of the switch reduced accordingly.